Electroacoustic Transducer

Abstract

An electroacoustic transducer of the magnetic-armature type including improved pivot mounting means for the armature. Elongated legs extend from the pivot end of the armature and provide a torsional force tending to restore the armature to its balanced or neutral position when the armature is vibrated.

Description

The inventive transducer comprises a small, direct radiator type of speaker and is particularly useful as a soft speaker, that is, it is a relatively small and compact transducer unit having a function intermediate or between that of an earphone and a loudspeaker. In one embodiment, the transducer includes a case which is about one inch square and three-eighths inch thick. The unit is relatively small but needs to have the capability of moving a relatively large volume of air; th at is, a relatively large movement of the armature and diaphragm is required. For comparison purposes, it might be pointed out that for certain present earphone transducers operating on similar basic principles, the movement of the armature is in the .+-.0.001 inch range while in the structure of the present invention the armature motion is .+-.0.010 inch range, i.e., a factor of ten times greater.

Because the movement of the armature is larger in the present device, and because the armature has to actuate a diaphragm which in turn drives a relatively large volume of air, the characteristics of the armature system must be improved and an improved restoring force must be provided to return the armature to its neutral, or balanced position. More explicitly, as the armature vibrates between two permanent magnets, the invention provides a restoring force to return the armature to its neutral position.

Accordingly, it is a principal object of the present invention to provide an armature for a miniature or sub-minature acoustical transducer having improved means for restoring the armature to its neutral or balanced position.

It is another object of the present invention to provide an improved armature for an acoustical transducer providing improved pivot support moving means.

It is still antoher object of the present invention to provide a gradual magnetic saturation of the armature system to minimize armature clatter or ringing.

It is yet another object of the present invention to provide an improved mounting for an acoustical transducer motor assembly.

The foregoing and other features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings, wherein:

FIG. 1 is a perspective view, partly in section, of the acoustic transducer of the invention;

FIG. 1A shows in sketch form one position or location of the inventive transducer in an associated receiver;

FIG. 2 is a perspective view showing the transducer coil, permanent magnets and yoke mounted on the bulkhead or base place of the transducer;

FIG. 3 shows the structure of inventive armature system;

FIG. 3A is a view in cross section taken along the lines 3A--3A of FIG. 3;

FIG. 4 shows the mounting of pivot and of the armature;

FIG. 5 is a back view taken along lines 5--5 of FIG. 4 showing the wire member for retaining the armature in its position;

FIG. 6 shows the drive pin connecting to the armature;

FIG. 7 is a perspective view of an armature system showing means of trapping the armature and also dampening means;

FIG. 7A is a view taken along lines 7A--7A of FIG. 7 to show the spacing, as desired, between a trapping member and the associated leg determining a translation constraint;

FIG. 8 is a cross-sectional view showing the motor assembly including the armature, diaphragm, permanent magnets and yoke mounted in the associated case;

FIG. 9 is a view showing the diaphragm in cross section;

FIG. 9A shows the diaphragm mounted on its associated wafer support;

FIG. 10 shows a modification of the inventive armature;

FIG. 11 shows another modification of the inventive armature;

FIG. 11A shows another modification of the inventive armature; and,

FIG. 12 shows a modification of the mounting for the pivot end of the armature.

Referring to the drawings, FIG. 1 shows the inventive transducer 11, having the case and cover thereof cut away to better show the motor assembly 14. FIG. 1A indicates one position or location of the transducer 11 of the invention as used in one application such as with an associated receiver 13 used in paging personnel, say in a hospital. The receiver 13 is carried as in a coat pocket of the user, and a selected signal activates the receiver 13 to page the user. It is understood that the receiver 14 might also be modified to function as a transmitter-receiver which could be used in two-way communication. The receiver 13 and transducer 11 while relatively small must be relatively sensitive and must also provide a strong audible signal.

The motor assembly 14 of the transducer 11 includes an electrical coil 21, armature 23, permanent magnets 33 and 35, and yoke 37 mounted on a bulkhead or base plate 15 having a pair of apertures 17 and 19. For purposes of clarity, FIG. 2 separately shows the base plate 15 including a part of the motor assembly 14 namely, the coil 21, the yoke 37 and the permanent magnets 33 and 35, which are positioned to form an air gap 36 therebetween. As best seen in FIG. 1, aperture 19 is arranged to accommodate the electrical coil 21, and aperture 17 communicates the space between a diaphragm 63 and the plate 15 to the back cavity.

Referring now to FIG. 3 as well as FIGS. 1 and 2, the aperture 23 has a free or vibrating end portion 25 and a pivoting end portion 27 which connects or extends into a pair of legs 29 and 31 which are transverse of the longitudinal axis of armature 23. The free end 25 of the armature is positioned in air gap 36 between the spaced permanent magnets 33 and 35. The one magnet 33 is mounted on the base plate 15 and the other magnet 35 is mounted on yoke 37 which is formed to have a flat top 39, two sides 41, and two pads 43 and 45 which bear on base plate 15. The front and back of yoke 37 are open to permit free movement of the armature 23. The armature 23 thus extends through the center of coil 21 and is balanced at a neutral position between the magnets 33 and 35.

In a quiescent condition, the armature 23 is maintained in a balanced or neutral position between the permanent magnets 33 and 35. An electrical signal from the receiver 13 applied to the coil 21 causes armature 23 to vibrate in response thereto. The vibration of the armature 23 is translated through drive pin 47 to the diaphragm 63 to generate acoustical energy. Likewise, sound waves impinging on diaphragm 63 will be translated through drive pin 47 to actuate armature 23 to generate an electrical signal in coil 21.

Referring now to FIG. 6 as well as FIGS. 1 and 3, the free end 25 of armature 23 is slotted as at 49 to receive one end generally labeled 48 of a drive pin 47. The end 48 of drive pin 47 includes a pair of spaced shoulders 48A and 48B forming a recess 48C therebetween which is received in slot 49 of armature 23. For purposes of convenience in manufacture and assembly, the other end 51 of drive pin 47 is similar to end 48 such that the ends can interchangeably fit into the slot 49 in armature 23.

The construction of the transducer 11 thus includes the advantage and having an end-coupled armature 23. The effective mechanical mass at the connecting drive point of the armature 23 of the invention is several times less than mass of a similar armature coupled near its mid point. Accordingly, an end-coupled armature which is heavier or larger in cross section may be utilized when designing for a given acoustical property. The larger armature provides increased flux-carring capability and also increased resistance to mechanical shock. The end-coupled armature also produces an increased volume displacement of the diaphragm over a near center-coupled armature. The associated diaphragm obtains maximum excursion due to the increase in distance from the pivot point of the armature to the connection of the drive pin.

Referring to FIGS. 4 and 5, the pivot end 27 of the armature 23 is constrained against movement in a direction perpendicular to its plane (see FIGS. 4 and 5) by a wire clip 67 which elastically clamps the armature end 27 on pivot support 61 which in turn is affixed as by spot welding on base plate 15.

As best shown in FIG. 5, the body portion of wire clip 67 extends over armature and the ends of wire clip 67 are bent under support 61 to hold the pivot end of armature 27 on pivot support 27. To prevent a metal-to-metal contact between the armature 23 and clip 67, and between armature 22 and pivot support 61, a compliant elastomeric shim 69 is placed between the armature 23 and clip 67 and between armature 23 and pivot support 61. The armature 23 is thus held on the pivot support by an elastic force. Note that the free end 25 of the armature 23 can move to function as a lever or crank about the pivot end 27 and does not depend for its movement on bending as a cantilever beam.

The legs 29 and 31 extend transversely of the longitudinal axis indicated by the dotted line 26, and in the embodiment of FIG. 3, the legs 39 and 31 which are formed in the plane of armature 23, are curved to join and form a ring 30 having a anchor point 55 diametrically opposite the armature 23 pivot support. Anchor point 55 is positioned on a spacer washer 57 to thus space legs 29 and 31 relative to base plate 15, and the anchor point and washer are affixed to the plate as by a suitable screw 58.

Note that in the construction shown in the various figures, the pivot point of the armature is near th periphery of the diaphragm. Such construction permits a relatively large coil and magnet assembly with its attendant advantage to be utilized in the transducer 11.

A small direct radiator-type acoustic speaker such as the inventive unit possesses a relatively uniform useful sensitivity above the resonant frequency of the diaphragm-motor assembly system; that is, the resonance frequency is at the low end of the system pass band. In this frequency region, the signal tractive force is opposed principally by the inertia of the armature and the diaphragm. At frequencies below this resonance, the magnetic forces are opposed by the elastic restraint on the armature and this establishes neutral or static position of the armature. For maximum performance in th useful frequency range, the masses must be kept small in proportion to the magnetic forces developed. In accordance with the foregoing, a principal object or feature of this invention is to provide an elastic restoring force with a minimum increase in the effective inertia of the diaphragm-armature combination.

As the armature 23 is vibrated, that is, as the free end 25 of armature 23 is caused to move up and down, a twisting or torsional stress is placed on legs 29 and 31. The torsional stress developed in legs 29 and 31 is distributed as a strain along the arc of these legs producing a rotation on each leg distributed approximately uniformly along its length. This distribution of the strain permits an appreciable energy to be stored without reaching the elastic limit of the material thus reducing the likelihood of mechanical fatique of this armature system.

Since the material in the legs 29 and 31 is all relatively near the center of rotation, the legs add minimal effective mass to the armature vibration mode while yet providing an effective torsional restoring force. For such desired torsional effect, the diameter of the ring 30 defined by legs 29 and 31 must be much larger than the cross-sectional dimensions of the legs 29 and 31.

The circular or ring construction of legs 29 and 31 enables the armature system 22 including armature 23 and legs 29 and 31 to be trapped or constrained to vertical motion such as by trapping members 66 and 68, see FIG. 7, limiting the maximum motion in the translational mode while yet permitting the legs to have minimum constraint in the torsional mode. This construction permits effective protection against damage by mechanical shock.

Also, dampening blocks such as 70 and 72 in FIG. 7 may be conveniently provided to dampen movement of legs 29 and 31 as desired.

As mentioned above, another feature of the inventive transducer is that armature clatter or ringing is reduced by providing a gradual or soft saturation of the armature system 22. The theory or function in providing a gradual of soft saturation of the armature system is believed to be as follows

A path for a static or d.c. flux can be traced in FIGS. 1 and 2 from magnet 33 across the air gap 36 and armature 23 to magnet 35, yoke 39 and dividing to sides 41 and 42 of the yoke, and back to magnet 35 by way of the plate 15.

A path for a dynamic or a.c. flux may be traced from coil 21 through the pivoted end 27 of armature 23, dividing to extend through legs 29 and 31, anchor point 55, support 57, plate 15, magnet 33 and the free or vibrating end of armature 23, and back to coil 21. A second a.c. flux path may be traced from coil 21 through the fixed end or armature 23, dividing to extend through legs 29 and 31, attachment 55, support 57 plate 15, through pads 43 and 45 and sides 41 and 42 of yoke 37, magnet 35, the free end of armature 23, and back to coil 21.

It is the interaction or combination of the aforementioned magnetic flux in the air gap 36 region that produces the useful unbalance of tractive force in the air gaps as current in the coil 21 is modulated.

As mentioned above, the legs 29 and 31 which are integral with, and of the same material as armature 23, are of relatively smaller combined cross-sectional area than armature 23. As shown in FIG. 3A, the cross-sectional area of the armature 25 is greater than the combined cross-sectional area of the two legs 29 and 31. Accordingly, as current in the coil 21 is increased to increase the magnetic flux in the armature 23, the legs 29 and 31 saturate before the armature saturates. As the current is further increased, the flux along the armature 23 must fing its return path through the air gap between the region at pivot end 27 (the juncture or armature 23 and legs 29 and 31) and the base plate 15. The flux flow tends to decrease gradually rather than to cease abruptly, thus there is a relatively gradual or soft saturation of the flux path. In other words, the legs 29 and 31 saturate before the armature 23 saturates, reducing the rate at which the tractive force is increased, before the armature 23 strikes one of the permanent magnets. The foregoing reduces armature clatter or ringing caused by the armature striking the magnets.

Referring to FIG. 9, the edge of base 97 of diaphragm 63 is affixed to a thin elastomeric polymer which forms a flexible surround 71 which in turn is affixed to a thin plastic wafer 64 which is affixed to the under side (as oriented in FIG. 7) of base plate 17. As iw well known, surround 71 permits the diaphragm 63 to properly move or flex.

The diaphragm 63 comprises a cone-shaped member having surfaces 95 and 97 formed with an outer skin of metal or plastic such as of Mylar. The interior of the diaphragm 63 is filled with hollow plastic microspheres 99. As is known in the art, microspheres 99 lightly coated with adhesive are then caused to adhere to one another to form a lightweight through rigid mass. The diaphragm 63 is thus light in weight and yet is relatively rigid.

The free end of drive pin 47 is inserted in the mass of microspheres 99 and affixed thereto as by cementing. The spaced shoulders 48A and 48B on end 53 provide means for anchoring drive pin 47 in the mass of microspheres 99. As mentioned above and as shown in FIGS. 1 and 7, the other end of drive pin 47 is affixed to the armature 23 and thus connects the armature 23 to the diaphragm 63.

Referring to FIG. 8 as well as FIG. 1, the transducer 11 includes essentially a cup-shaped case 75 having upstanding side walls generally labeled 77. A cover 79 fits inside walls 77 on suitable locating shoulders 81 and is affixed to walls 77 as by cement. As clearly shown in FIG. 8, the base plate 15 including the motor assembly 14 is fitted at an angle in case 75. One pair of edges of base plate 15 rests on suitable locating projections labeled 85, and plate 15 is affixed in position by cementing. The other edges of plate 15 rest on the side walls (as oriented in FIG. 8) of case 75 and affixed thereto by cementing as at 88.

Accordingly, the positioning of the base plate 15 in case 75 thus forms a front cavity 87 which diaphragm 63 separates from a back cavity 89 wherein motor assembly 14 is mounted. The front cavity 71 opens to the atmosphere through a sound port 73. The case 11 also includes a back cavity 89 containing the motor assembly 14 which is vented through port 76 to the receiver case 13. As is known, by venting the back cavity to a relatively large volume of air, the response of the transducer to low frequency sound is improved. In the present construction to improve or smooth out the response of the system over the range of interest, a baffle cloth 78 is placed over the port 76.

Positioning the base plate 15 at an angle permits the sound ports 73 and 76 to be of maximum size for the dimensions of the wall used.

FIG. 10 shows a modification of the armature system of FIG. 3. In the modification of FIG. 10, the armature 23A is similar to the armature 23 of FIG. 3. However, in FIG. 3 the legs (one only being shown in FIG. 10, with the other leg being a mirror image there) are formed in a plane relatively perpendicular to the plane of the armature 23A. In addition, flange 91 extends from the pivot end of armature 23 to the anchor point 55A. A pirncipal advantage of the construction of FIG. 10 is in the fabrication or manufacturing process. In the structure of FIG. 3, to change the characteristics of the armature system 22, the dimensions of the ring 30 may be changed; however, this may involve in change in the stamping die employed. In the structure of FIG. 10, the characteristics of the armature system may be changed by trimming the top surfaces of the legs as at 93 which is a relatively more simple and efficient process.

FIG. 11 shows a second modification of the armature system 22 wherein the legs 29B and 31B are similar to legs 29 and 31 of FIG. 3. However, legs 29B and 31B each extend only as an arc of a ring and are affixed to plate 15 at suitable spaced supports 54A and 54B each similar to anchor point support 55. The structure of FIG. 11 provides different output characteristics from that of the structure of FIG. 3.

FIG. 12 shows a modification of the pivot support structure shown in FIG. 4. In certain instances, the structure of FIG. 4 may subject to a scrubbing action, that is, slight back and forth movement along the longitudinal axis of the armature 23. The foregoing scrubbing action may be minimized by forming a shoulder 24 in armature 23. Thee shoulder 24 accommodates or receives the top of pivot support 61 and the compliant material 69 at approximately the thickness center line of the armature and arcuate legs.

Another modification of the inventive armature system useful in certain applications is shown in FIGS. 11A. In FIG. 11A, the shape of the armature 123, and more particularly the pivot end of the armature, is similar to that of armature 23 of FIG. 12. The pivot end of armature 123 is affixed to legs 129 and 131 which may comprise, for instance, a length of spring material suitably affixed at a mid point thereof to the pivot end of armature 123. Legs 129 and 131 are formed in an open ring configuration sigmilar to the structure of FIG. 11. The ring configuration of FIG. 11 has a large radius of curvature relative to the cross section of the legs similar to the structure shown in FIGS. 3, 10 and 11. The ends of legs 129 and 131 are suitably affixed to the associated base plate 15 as at 143 and 144 indicated in FIG. 11.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

We claim:

1. An electroacoustic transducer having an armature system wherein soft saturation of the armature system is obtained to reduce armature clatter or ringing caused by the armature striking the magnets, the armature system including an elongated magnetic armature having a free end and a pivoting end, magnets and coils assembly means for causing the armature to vibrate from a static and neutral position, a base plate, a support on the base plate for mounting the pivot end of the armature, flux conductive legs extending transversely from adjacent the pivot end of the armature to develop a force tending to restore the armature to its neutral position, the cross sectional dimensions of the legs being less than the cross sectional dimensions of the armature, a first flux path for the armature system traceable from the pivot end of the armature through the legs, the base plate, the magnets and to the free end of the armature to provide a low reluctance metallic path for flux flow, the legs magnetically saturated before the armature saturates, and a second and higher reluctance flux path being established from the pivot end of the armature through the air gap between the region of the pivot end and the base plate when the first flux path saturates, whereby the flux flow through the armature system tends to decrease gradually to provide a gradual or soft saturation of the armature system to thereby minimize armature ringing.

2. An electroacoustic transducer as in claim 1 wherein an end of each leg is in flux-conductive contact with the base plate to provide a flux path in series with a flux flow path of the armature.

3. An electroacoustic transducer as in claim 1 including elastic means for holding the armature on the pivot support, and compliant means positioned between the armature and the pivot support and between the armature and the elastic means to thereby prevent a metal to metal contact.

4. An electroacoustic transducer as in claim 1 wherein the legs are of the same cross-sectional dimension and the cross-sectional area of the armature is larger than the combined cross-sectional area of the legs, and the legs are formed as a ring having a large radius of curvature relative to the cross-section of the legs whereby the legs can provide desired torsional forces while permitting maximum constraint to translation of the legs.